In a conventional semiconductor device having an HJFET (hetero junction field effect transistor) of an AlGaN/GaN structure, in general, an electrode of a Ti/Al structure has been used as an ohmic electrode (see, for example, Patent Document 1).
However, in the electrode formed of the Ti/Al structure alone, if heat treatment is performed at a temperature greater than a melting point (660° C.) of Al to reduce the contact resistance of the electrode, the surface coating ratio of an electrode material to a semiconductor film decreases due to the surface tension of the molten Al. As a result, realization of a low contact resistance tends to be difficult.
To overcome this problem, it has been reported to use an electrode having a Ti/Al/Mo/Au structure where the surface of Al is coated with Mo (see, for example, Non-Patent Document 1). In the electrode having a Ti/Al/Mo/Au structure, since the coating ratio of a semiconductor film to an electrode material is not reduced even if a heat treatment is performed at a temperature of equal or greater than 800° C. exceeding the melting point of Al, the low resistance can be realized compared to the electrode having a Ti/Al structure.    Patent Document 1: Japanese Patent Laid-Open No. 7-248204    Non-Patent Document 1: Kumar et al., Journal of Applied Physics, Vol. 92, No. 3, p. 1712